That much is true. I found this a long time ago while browsing Dogasu's Backpack:
ichael Haigney: "In an early episode, there was a new...new character, and it was Charmander. So, at the end of the session, all the major a-actors were done, and Charmander had to make some kind of a sound. So I thought "well...OK here's...here's one Pokémon, there are a hundred and fifty. We have fifty-two more episodes. Charmander's probably in this one episode and we'll never see Charmander again. [the other actors laugh] So I say...y'know, we had like a little time...so y'know what? I'm just gonna jump in, and I'll do a sound. And it was like [in Charmander's voice] "Char! Char! Char!" like that. And...'cuz he was a little character. And [in Charmander's voice] "Chaaaaar!" like that. [the other actors laugh] OK, so I thought I was done."
Norman Grossfeld: "Wait...a little...one more time." [the other actors laugh]
Veronica Taylor: "How's it go again?"
Michael Haigney: "So then... [in Charmander's voice] Char! Char!"
Maddie Blaustein: [in Damien's voice] "It's Charminder!" [the other actors laugh]
Michael Haigney: "Right...it was early. Right, a-a-a-a-and Maddie was in it that episode I think. So...then like we got a script..."OK...now Ash, Ch-...and here's Charmander again. And I thought "Oh maybe Charmander's in a two-parter." And then it was Charmander for..."
Veronica Taylor (?): "A long time."
Michael Haigney: "...like...seventy-seven episodes, it seems like. And that started happening...and so...y'know we never really knew when a...when a...when a Pokémon was gonna be in a lot of episodes..."